Ring-strapped multifilar helix



Nov. 22, 1960 R. P. LAGERSTROM EI'AL 2,961,572

RING-STRAPPED MULTIFILAR HELIX Filed July 2]., 1959 FIG.../

INVENTORS RICHARD LAGERSTROM DONALD A. DUNN ev t United States Pater 2,961,572 RING-'STRAPPED MULTIFILAR HELIX Richard P, Lagerstrom, Palo Alto, and Donald A. Dunn, Menlo Park, Calif., assignors, by mesne assignments, to the United States of America as represented by the Secretary of the Navy Filed July 21, 1959, Ser. No. 828,674

Claims. (Cl. SIS- 3.6)

This invention relates to a slow-wave structure for use in traveling-wave tube amplifiers, and more particularly to a ring-strapped multifilar helix.

An object of this invention is to provide a slow-wave structure which, when used with an appropriate highvoltage electron beam (for example to 40 kilovolts) will form a traveling-wave tube amplifier with the following characteristics: large 3-db bandwidth from about 1.25:1 to 1.5: 1; and the tube will be relatively free from self-oscillation caused by beam-interaction with a backward space-harmonic of the structure having a velocity nearly synchronous with the electron beam, as compared with a single helix having a similar beam configuration and equal pitch and diameter.

Other objects and many of the attendant advantages of this invention will be readily appreciated as the same becomes better understood by reference to the following cle-' tailed description when considered in connection with the accompanying drawing wherein:

Fig. l is a more or less schematic drawing of a travelingwave tube, showing one preferred embodiment of the invention;

Fig. 2 is an enlarged axonometric view of a ringstrapped bifilar helix removed from the tube of Fig. 1; and

Fig. 3, is a cross-section taken along the line 3--3 in Fig. 1.

Reference is now made to the drawing. The particular multifilar helix illustrated is a bifilar helix made of two helices 2 and 4 of uniform conducting tape of width W and thickness ,T. Each has the form of a helix of constant pitch p (the axial the distance in which the conductor makes one complete rotation about the axis) constant pitch angle 0 (the complement of the angle between an element of the cylindrical surface in which the helix lies and the helical direction in that surface), and constant mean radius a. The helices are spaced equally from each other along their common axis.

The helices are connected together electrically (strapped) every distance d by means of conducting circular rings 6 of (axial) width w and (radial) thickness t, shown in Fig. 2 to be the same thickness as the thickness of the helical tapes. The rings 6 are coaxial with the bifilar helix and are shown in Fig. 2 as being of the same mean radius as the helical conductors.

The helices and rings (straps) can be the same piece of metal formed into the shape described, or they can be separate pieces of metal formed as described and connected together to afford electrical connection and physical support between the separate pieces.

The cross-sectional shape of the conductors which form the helices may or may not be the same shape as the conductors which form the rings. These shapes can be tapes, as illustrated in Fig. 2, or they can be other sections such as circular, oval, square, etc. The configurations of the conductors constitute the essential feature which determine the characteristics of the structure while the cross-sectional shape of the conductors is of secondary concern.

The traveling-wave tube in which the helix of this invention is used is indicated generally at 8 and is of conventional construction including an electron gun 10, collector 12, input terminals 14, and output terminals 16. The electron beam travels generally along the axis of the helix. The helix can be supported by dielectric supporting rods inside of it (omitted for the sake of clarity) or can be supported by the connecting terminals alone if the tube is disposed with its axis vertical.

-An understanding of the operation of this invention can perhaps best be had by review of the operation of an unstrapped bifilar helix. The unstrapped bifilar helix has an infinity of possible modes of propagation, two of which are important here. The symmetric or pushpush mode is characterized by fields at the two helices being in phase with each other. It has a forward spaceharmonic (designated as a fundamental or (0) spaceharmonic) which is useful for forward wave amplifiers, and a strong -2 backward space-harmonic which can cause oscillations when its velocity is nearly synchronous with the electron beam and therefore also nearly synchronous with the velocity of the fundamental spaceharmonic.

An anti-symmetric or push-pull mode is also possible, for which the fields at the two helices are out of phase at any axial position, even at very low frequencies. This mode has a strong 1 backward spaceharmonic which can cause oscillations under the same near-synchronous condition with the beam and therefore also with the fundamental space-harmonic. The 1 and the O space-harmonics are synchronous at or near the frequency for which the circumference of the structure is equal to one-half of a free-space wavelength. Recalling that a is the mean radius of the structure, the terminology common in the art defines ka as the circumference measured in free-space wavelengths. Thus, the aforementioned synchronism of the 1 and 0 spaceharm-onics, can be said to occur near ka=l.0.

The straps of the present invention connect the two helices together electrically, thereby forcing them to be in phase and prevent a push-pull" mode with its -1 space-harmonic from propagating. The straps, because of their periodic perturbation of the unstrapped structure, also cause a stop-band in the symmetric mode. By suitable choice of the strap spacing d there is produced a, stop-band which results in considerably higher beam currents being required for oscillation in the strapped structure than for interaction with the -2 space-harmonic ofthe unstrapped structure. The stop-band is needed at about ka=l.() which allows forward-wave interaction (traveling-wave tube operation) up through ka=0.5 and higher. Operation through such a high value of ka is one requirement for broadband amplification in a high power tube, the other requirement of the structure being low dispersion.

An explanation of some aspects of the high power amplifier design problem that lead to the aforementioned requirements on ka and voltage is given herewith to facilitate an understanding of the need for and function of the present invention. Items of particular concern are high power, bandwidth, power gain, and backward-wave oscillations. A short discussion of each will make their inter-relation apparent.

The main necessity for high power output. aside from low loss on the slow-wave circuit, is that there be high power in the electron beam. There is a limit on the desirable beam perveance (1 X10" for a solid beam) so that a high power beam must also be a high voltage beam. For the circuit wave to interact with such a fast,

beam, the phase velocity of the wave (a forward spaceharmonic) must be a relatively large fraction of the velocity of light, which implies a relatively fast circuit, compared to the circuits used with low voltage beams.

The range of frequencies for approximately constant gain and power output is determined by two conditions. (1) The beam and wave velocity must be nearly synchronous over the desired bandwidth of the tube. The beam velocity is fixed, so the circuit must have a wide range of frequencies over which its fundamental spaceharmonic has essentially constant velocity (low dispersion). (2) The helix radius times the axial propagation constant is a fixed number (between 1 and 2) at the peak of the gain curve in a solid beam amplifier. Because this number, 7a, is fixed, and because the phase velocity is proportional to ka/va, the value of ka must be increased as the voltage of the beam is increased. For beam voltages of 10-40 kilovolts ka must be as high as 0.5.

The gain of a traveling-wave tube increases with the beam current, the strength of the interaction between the currents and fields of the beam and of the circuit (quantitatively, the interaction impedance), and with the length of the interaction region. For forward-wave amplification, which is desired, the circuit wave of interest is a forward space-harmonic near synchronisrn with the beam velocity.

The circumference of high velocity circuits may be on the order of a free space wavelength. When this is true, there often is a backward space-harmonic component (of some permissible mode of the circuit) which will have a high beam interaction impedance. Such an impedance, if it occurs at a frequency for which this backwardharmonic has a phase velocity nearly in synchronism with the beam, may allow backward wave oscillations to build up at the synchronous frequency. The conditions for the start of such oscillations are identical to those for realizing good power gain in the forward spaceharmonic, i.e. high beam current, high impedance, and some minimum length at a nearly synchronous velocity. Oscillations are inimical to good amplifier operation, so the conditions for oscillations must be selectively defeated. In a circuit for a high power amplifier, it is important, then, that near the synchronous velocity a backwardharmonic either does not propagate, or has a very low impedance compared to the fundamental impedance.

The criteria for a suitable high power traveling-wave tube circuit are principally that the circuit have a reasonably non-dispersive region about a velocity corresponding to a high voltage beam when the circumference of the structure is about half free space wavelength, and that there be no backward space-harmonics having appreciable impedance (compared to forward impedance) at this velocity. Multifilar ring-strapped helices are considered to meet these criteria. Although the preferred illustrated embodiment is a bifilar helix, multifilar helices using more than two identical helices assembled in intertwined relation along a common axis equally spaced from one another are comprehended Within the scope of this invention. As the helices become more numerous, the width of each one would become smaller so that the individual helices do not touch each other.

The ring strapping of multifilar helices containing more than two helices can be accomplished in the same man ner as with the bifilar helices, since the rings can come in contact with all the separate helices at any chosen axial position. The strapping rings need not be of the same mean diameter nor of the same size nor shape of cross-section, as the conductors which form the individual helices. Thus, the rings could be entirely inside of the helix conductors (hence have a smaller mean diameter) with electrical contact made at the inside surface of the helix conductors and the outside surface of the rings. The rings could also be outside of the helices.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.

What is claimed is:

1. A slow-wave structure for a traveling-wave tube comprising a multifilar helix including a plurality of separate helices, each of the separate helices being of uniform conducting material, being of the same constant pitch angle, and constant mean radius; said separate helices being spaced equally from each other along a common axis; and a plurality of conducting circular rings coaxial with the multifilar helix electrically connecting the separate helices together and spaced periodically in the axial direction of the helix.

2. The slow-wave structure of claim 1 wherein the multifilar helix is a bifilar helix.

3. The slow-wave structure of claim 2 wherein the rings have the same mean radius as the helical structures.

4. The slow-wave structure of claim 1 wherein the rings are axially spaced from one another a distance equal to one-half the pitch of each separate helix.

5. In a traveling-wave tube, an electron gun, a collector for electron, and a slow-wave structure, said slowwave structure being constituted by the structure of claim 1.

References Cited in the file of this patent UNITED STATES PATENTS 2,836,758 Chodorow May 27, 1958 FOREIGN PATENTS 668,017 Great Britain Mar. 12, 1952 

